tictactoeFX
A not-your-everyday tic tac toe implementation that allows to go beyond the standard format. You can choose to increase the size of the board (3x3, 4x4, 5x5,...) and the amount of players (2, 3 or 4). The idea of this is to give this classic a new spin.
Written in JDK8 and JavaFX, it will, unfortunately, take some efforts to make this fascinating free time project accessible again. Until then, you can checkout its its software architecture documentation and dive deeper into the source code. It features some interesting design patterns typically used in object oriented programming.